drm/ttm: Don't override vm_ops callbacks, if set

Drivers may want to set their own callbacks for a VM area. Only set
TTM's callbacks if the vm_ops field is clear.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-2-tzimmermann@suse.de